Photoelectric cell reproducer



Oct. 1,1939. I HB-ELAR 2,178,219

PHOTOELEGTRIC CELL REPRODUCER Filed May 20, 1937 +90 VOLT5 ISnvento:

Evin/53,621

7 g (Ittomeg Patented Oct. 31, 1939 UNITED STATES PATENT OFFME Herbert Belar, Philadelphia, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application May 20, 1937, Serial No. 143,716

18 Claims.

This invention relates to sound reproducers and more particularly to sound reproducers which include. a photoelectric device for picking up variations in light intensity caused by the sound record.

There have, heretofore, been many types of photoelectric cell reproducers. One of the types recently introduced into commercial use operates with a film sound record having two opposed recordings of the sound waves 180 out of phase with each other, commonly referred to as a Class A push-pull record.

In my application Serial No, 52358, filed December 2, 1935, now Patent No. 2,152,173, issued March 28, 1939, there is disclosed and claimed a number of circuits for push-pull sound reproducers including change-over means so that either a push-pull record or a single sound record could be played selectively on the sound reproducer. The present invention is an improvement on the apparatus of my prior application.

In the apparatus or" the prior application, either the two photocells were connected at opposing ends of a center tapped resistor, transformer or equivalent network, or both were connected to the same end of the transformer or other translating device, leaving one-half of it inoperative for reproduction of single sound records. I have discovered that such a circuit may cause a change in volume when a changeover is made from a push-pull record to a similar single record. This change in volume is undesirable.

In the circuit according to the present invention, I avoid any volume change when shifting from push-pull to single sound track operation, or vice versa. This is accomplished by so connecting the photocells that they at all times work across the entire primary winding of the transformer and are connected thereto by an appropriate capacitance and resistance network so that neither the frequency response nor the amplitude suffers any change except that due to the records themselves.

4,5 One object of my invention is to provide a novel method of and apparatus for changing from push-pull sound reproduction to single sound reproduction, or vice versa.

Another object of my invention is to provide .50 such a change-over device wherein no change .55 the primary of the power supply transformer.

Another object of my invention is to provide such a device wherein the output circuits of the photocells may be readily balanced.

Another object of my invention is to provide a relatively simple and inexpensive circuit for "5 changing over from single to push-pulloperation with a minimum of circuit elements.

Other and incidental objects of my invention will be understood by those skilled in the art from a reading of the following specification and 9 an inspection of the accompanying drawing in which:

Figure 1 is a schematic diagram of my photocell circuits up to and including the output transformer, 5

Figure 2 is a simplified schematic diagram of the circuit of 1 with the change-over switch in the position for operation with a single sound track, and

Figure 3 is a simplified schematic diagram of 20 the circuit of Fig. 1 with the change-over switch in the position for operation with a push-pull sound track.

The system of Fig. 1 includes photocells l0 and H which are, of course, arranged in proper 2 5 relation to an'appropriate optical system in the usual type of soundhead, as will be readily understood. These photocells are subjected to an appropriate potential through the balancing network hereinaiter described and through the re- .30

'spective resistors l5 and I6. Each of the photocells is connected to the output transformer 24- through a coupling condenser l3 and it, respectively, which are so chosen that their reactance is small compared to the reactance of the pri ,3'5 mary of the transformer 24-. The resistors IE5 and 16 are chosen so that their impedance is large with respect to the impedance of the transformer primary. 'A switch !2 is provided so as to connect the tubes it and H through the condensers l3 and M to the primary 24 in either push-pull or parallel relation, as desired, the parallel relation shown in Fig. 2 being provided when the switch is in the position indicated at and the push-pull relation of Fig. 3 being provided when the switch is in the position 3.

A balancing potentiometer 22 is connected to the current supply leads as a portion of the balancing network. For compensating for the characteristic of the photocells there are included the resistors 2! and 23 which may be made adjustable if desired. The filter network includes the resistors 19 and 2B and the capacitors ii and. I8 which are provided for preventing any noises due to the operation of the potentiometer 212 iromv reaching the photocell circuits and thereby reaching the output of the device.

As a practical example of a commercial embodiment of my invention, the transformer 24 may have a primary inductance of 3,000 henrys; the secondary of this transformer may have an inductance of 7.5 henrys; and the connected load from the secondary may be such that the impedance looking into the primary of the trans former is 200,000 ohms over the entire audio frequency. In this case the capacitors I 3 and I4 would have a value of .1 microfarad and the resistors l5 and I6 would have a value of 1 megohm. The capacitors I3 and M will, therefore, have at the lowest audio frequency to be handled, i. e., 30 cycles per second, a reactance of about 50,000 ohms or approximately onefourth of that of the primary of the transformer 24, while the resistors l5 and [5 will have a value of approximately five times that of said primary.

With the foregoing arrangement, when the switch I2 is in the position 2, the photocells are connected to the transformer in the manner shown in Fig. 2, while, when the switch is moved to the position 3, they are connected in the manner shown in Fig. 3 without any change in output or frequency characteristic of the circuits.

The transformer 24 merely typifies an output circuit and the leads connected thereto may be connected to any equivalent output circuit or device.

It will be apparent to those skilled in the art that various changes may be made in this circuit in accordance with the scope of my invention such, for example, as substituting reactors for resistors, changing the position of the capacitors in circuit, or the like. It will also be apparent that my invention is not limited to the use of photocells but that it is equally applicable to any equivalent circuit wherein a pair of devices or circuits are used which are connected to a D. C. power supply and have an A. C. audio frequency or radio frequency output and which are intended to be connected in either parallel or push-pull selectively. The pair of photocells need not be enclosed within separate envelopes, but may, for example, be enclosed within a single envelope, as in the RCA Radiotron No. 920 twin phototube.

Having now described my invention, I claim:

1. A photocell circuit including a pair of photocells, an output circuit having two leads, coupling means connecting each of said photocells to said output circuit, the said means having a reactance small compared to that of the output circuit, means for supplying current to said photocells, and means for selectively coupling said cells to the output circuit leads either in parallel or push-pull relation.

2. A photocell circuit including a pair of photocells, an output circuit having two leads, coupling condensers connecting each of said photocells to said output circuit, the said condensers having a reactance small compared to that of the output circuit, means for supplying current to said photocells, and means for selectively coupling said cells to the output circuit leads either in parallel or push-pull relation.

3. A photocell circuit including a pair of photocells, an output circuit, coupling means connecting each of said photocells to said output circuit, the said means having a reactance small compared to that of the output circuit, means for supplying current to said photocells, means for selectively coupling said cells to the output circuit either in parallel or push-pull relation, and

means for balancing the outputs of said photocells.

4. A photocell circuit including a pair of photocells, an output circuit, coupling condensers connecting each of said photocells to said output circuit, the said condensers having a reactance small compared to that of the output circuit, means for supplying current to said photocells, means for selectively coupling said cells to the output circuit either in parallel or push-pull relation, and means for balancing the outputs of said photocells.

5. The combination of a pair of light responsive devices, a pair of power input leads, impedance elements connected in parallel between said leads, means for applying the potential of one of said elements to one of said devices and for applying the potential drop of the other of said elements to the other of said devices and pair of output leads, and means for coupling said elements to said output leads in similar or opposed polarity relation.

6. Apparatus of the class described comprising a pair of A. C. output leads, two A. C. input circuits each having a D. C. shunt feed. and

means for selectively and simultaneously connecting said input circuits either in parallel or push-pull across said output leads.

'7. Apparatus of the class described comprising a pair of audio frequency output leads, two audio frequency input circuits each having a D. C. shunt feed, and means for selectively and simultaneously connecting said input circuits either in parallel or push-pull across said output leads.

8. Apparatus of the class described comprising a pair of A. C. output leads, two A. C. input circuits each having a D. C. feed, and means for selectively and simultaneously connecting said input circuits either in parallel or push-pull across said output leads.

9. Apparatus of the class described comprising a pair of A. C. output leads, two A. C. input circuits each having a D. C. shunt feed, means having a reactance small in relation to that of the 10. Apparatus of the class described comprising a pair of A. C. output leads, two A. C. input circuits each having a D. C. shunt feed, and means for selectively and simultaneously connecting said input circuits either in parallel or push-pull across said output leads, the said D. C. feeds having an impedance high in relation to that of said output circuit.

11. Apparatus of the class described comprising a pair of A. C. output leads, two A. C. input circuits each having a D. C. shunt feed, means having a reactance small in relation to that of the output circuit for connecting said input circuits to said output circuits, and means for selectively and simultaneously connecting said input circuits either in parallel or push-pull across said output leads, the said D. C. feeds having an impedance high in relation to that of said output circuits.

12. Apparatus of the class described comprising a pair of A. C. output leads, two A. C. input circuits each having a D. C. shunt feed, means for selectively and simultaneously connecting said input circuits either in parallel or push-pull across said output leads, and means for balancing said input circuits.

' circuits to said output circuits, and means for 13. Apparatus of the class described comprising a pair of audio frequency output leads, two audio frequency input circuits each having a D. C. shunt feed, means for selectively and simultaneously connecting said input circuits either in parallel or push-pull across said output leads, and means for balancing said input circuits.

' 14. Apparatus of the class described comprising a pair of A. C. output leads, two A. C. input circuits each having a D. C. shunt feed, means having a reactance of the order of one-fourth that of the output circuit for connecting said input selectively and simultaneously connecting said input circuits either in parallel or push-pull across said output leads.

15. Apparatus of the class described comprising a pair of A. C. output leads, two A. C. input circuits each having a D- C. shunt feed, means for selectively and simultaneously connecting said input circuits either in parallel or push-pull across said output leads, and the said D. C. feeds having an impedance of the order of five times that of said output circuit.

16. Apparatus of the class described comprising a pair of A. C. output leads, two A. C. input cir cuits each having a D. C. shunt feed, means having a ,reactance of the order of one-fourth that of the output circuit for connecting said input circuits to said output circuits, means for selectively and simultaneously connecting said input circuits either in parallel-or push-pull across said outputleads, and the said D. C. feeds having an impedance of the order of five times that of said output circuits.

17. Apparatus of the class described comprising a pair of A. C. output leads having a reactance of the order of 200,000 ohms, two A. C. input circuits each having a D.. C. shunt feed, means having a reactance of the orderof 50,000 ohms for connecting said input circuits to said output circuits, means for selectively and simultaneously connecting said input circuits either in parallel or push-pull across said output leads, the said D. C. feeds having an impedance of the order of one inegohm.

18. Apparatus of the class described comprising a pair of A. C. output leads, two A. C. photocell input circuits each having a D. C. shunt feed, and means for selectively and simultaneously connecting said input circuits either in parallel or push-pull across said output leads.

HERBERT BELAR. 

